rpc: Check that left section is not multiline

Summary:
PR description:

Motivation

RPCArg names in the rpc help are currently only used for documentation. However, in the future they could be used to teach the server the named arguments. Named arguments are currently registered by the CRPCCommands and duplicate the RPCArg names from the documentation. This redundancy is fragile, and has lead to errors in the past (despite having linters to catch those kind of errors).

Changes

The changes here add an assert in the CRPCCommand constructor that the RPCArg names are identical to the ones in the CRPCCommand.

This is a backport of core#19386 [1/5]
https://github.com/bitcoin/bitcoin/pull/19386/commits/fa8ec00061567e56333bb69c5623919d45a9a92d

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10006